Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

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Technique

Ultrasonography

Ultrasonography (US) has become the modality of choice for scrotal imaging. It can differentiate intrafrom extratesticular tumors with a sensitivity of over 95%. The specificity, however, is considerably lower. Transducers using 10-MHz provide detailed resolution, although with scrotal swelling a lower frequency is necessary for full coverage.

Seminal vesicles and surrounding structures are readily studied with endorectal US. Threedimensional (3D) endorectal US outlines the prostate in three planes and aids the study of transition zone hyperplasia; the central zone and enlarged transition zones are best identified using a coronal plane.

Transabdominal gray-scale US has a limited role in evaluating the prostate; the gland is located too far posteriorly and is too small for detailed study. An endorectal US approach is preferred.

Variability in interpreting endorectal US prostate images is a concern. Even well-trained physicians differ both in describing findings of random videotaped images of the prostate and in deciding whether to biopsy.

Transperineal US during contrast enhanced voiding urosonography has been proposed in children (1). Posterior urethral valves can be detected with this technique but further work is necessary to establish its clinical relevance.

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) of the scrotum is still evolving. Especially in children, with an equivocal US study MRI may obviate the need for exploratory surgery. In addition, new applications being reported suggest that MRI will evolve into the primary imaging modality of the testes.

Endorectal surface coils allow prostate zonal study. The prostatic capsule, neurovascular bundles, vas deferens, and seminal vesicles are readily studied.

T1-weighted magnetic resonance (MR) images outline the prostatic margin with surrounding fat, although the internal prostate zonal architecture is not defined. T2-weighted images show a hyperintense peripheral zone and a heterogeneous more hypointense central and transition zones. An endorectal coil improves spatial resolution. Postgadolinium T1weighted images reveal less peripheral zone enhancement than central zone.

Magnetic resonance voiding cystourethrography is feasible after gadolinium-enhanced excretory MR urography. Magnetic resonance fluoroscopy using a T1-weighted gradient echo sequence provides real-time urethral imaging during voiding. Both sonourethrography and MR imaging of the anterior urethra are feasible, with the urethral lumen distended with saline. Both modalities provide information about periurethral tissues.

Urethrography

At times urethral catheterization is difficult or even impossible. In such a setting, advancing a urethral catheter over a hydrophilic guidewire is helpful. Ultrasonography has been used instead of a retrograde urethrogram to visualize the urethra; US can provide a 3D view of the urethra.

Urethrography is performed using an iodinated contrast agent. Occasionally reported is use of undiluted gadolinium diethylene- triamine-pentaacetate (Gd-DTPA) in someone with a severe reaction to iodine-containing contrast, achieving good radiographic contrast, but such use should be balanced against the greater toxicity of gadolinium contrast compared to iodinated contrast.

Cavernosography

Cavernosography, performed by direct contrast injection into the corpora cavernosa, provides cavernous arterial systolic occlusion pressure. Vasodilators aid in interpreting these studies. This study currently is rarely performed, having been supplanted by US and clinical evaluation using erection-enhancing pharmacologic agents.

Color duplex Doppler US is considerably simpler than cavernosography and measures peak velocity flow, which correlates with cavernous arterial systolic occlusion pressure; peak velocity flow measurement represents a noninvasive evaluation of corporeal artery function.

Scintigraphy

Monoclonal antibody radioimmunoscintigraphy with indium-111–capromab pendetide is promising in men with prostate cancer, both to detect initial lymph node involvement and in those with suspected recurrent or residual disease after prostatectomy. This murinederived antibody is believed to act against the intracellular domain of prostate-specific membrane antigen, a glycoprotein expressed by prostate epithelial cells.

Testicular scanning is performed with technetium-99m (Tc-99m)-pertechnetate. This radiotracer establishes a blood flow pattern.

Biopsy

Traditional prostate biopsy is performed using a digital rectal examination for guidance, with a needle being inserted either transperineally or transrectally. Currently an endorectal USguided approach is commonly employed, using a needle up to 18-gauge for core biopsies. If a tumor can be palpated or localized by US, a direct biopsy is obtained, but, as will be discussed later, US-guided multisextant biopsies are often preferred. Endorectal US-guided prostatic nerve blockade increases patient comfort prior to systematic needle biopsy of the prostate.

A transrectal US-guided prostate biopsy is quite safe. Of necessity, a transrectal biopsy approach is through a nonsterile field, but postprocedure infections are surprisingly few.

An MRI-guided prostate biopsy is also feasible, mostly using an open magnet.

Vesiculography (Vasography)

Seminal vesicle, vas deferens, and ejaculatory duct patency is established by vesiculography, performed by cannulating and injecting contrast into the vas deferens. Surgical cannulation is often employed, although endorectal USguided seminal vesiculography is feasible. If endorectal US identifies dilated seminal vesicles or midline cysts, US-guided seminal tract puncture and contrast injection allows radiographic visualization of these structures. Injecting a mixture of contrast agent and indigo-carmine dye is useful if identification of these structures is desired during subsequent surgery.

Congenital Abnormalities

Prostate

Two types of prostatic ectopia are possible, developing from either urogenital sinus or mesonephric structures. Ectopic prostatic tissue has been found along the lateral rectal wall.

Müllerian remnants, including utricular cysts, coexist with hypospadias.

Wolffian Duct Structures

Failure of seminal vesicle buds to develop results in absent seminal vesicles. Some individ-

MALE REPRODUCTIVE ORGANS

uals also have ureteric bud failure, leading to an absent ipsilateral ureter and kidney. Thus detection of a seminal vesicle cyst or similar anomaly should lead to a renal study to exclude associated renal agenesis. Most seminal vesicle cysts are congenital; they are discussed later (see Wolffian duct structures).

Congenital bilateral vas deferens absence is a cause of azoospermia. This condition is detected with rectal US.

An ectopic ejaculatory duct insertion is rare.

Persistent Müllerian Duct Syndrome

Normally in a male fetus antimüllerian hormone causes müllerian duct regression. Absence or lack of function of this hormone results in a uterus and fallopian tubes being present in a phenotypically normal male, a condition called persistent müllerian duct syndrome. Two types exist: in the male type, one testis descends normally but the uterus and fallopian tubes are in or close to the inguinal canal, while in the female type bilateral cryptorchidism is evident and testes are located in the broad ligament and the uterus is in the pelvis. Simple cryptorchidism is generally suspected preoperatively because male external genitalia are otherwise normal.

Imaging findings are confusing if the diagnosis is not suspected. Computed tomography (CT) and MR identify a uterus and fallopian tubes as tubular structures posterior to the bladder, and testes located close to where ovaries are normally found in a phenotypically normal male. Testicular atrophy develops in older individuals, making further identification difficult. Similar to other males with cryptorchidism, these individuals are prone to developing testicular cancer.

Urethra

Duplication

Urethral duplication is rare. Most are located in the midline and lie in a dorsal-to-ventral direction. Duplications range from complete to incomplete, and the duplicated urethra originates either from the bladder or a normal urethra. An accessory urethral termination ranges from an epispadial to a hypospadial location. Some duplications are associated with

renal and multisystem abnormalities. Pubic bones tend to be abnormally separated with epispadial duplications.

A congenital urethroperineal fistula is a separate entity distinct from a duplication. Boys with a congenital urethroperineal fistula have normal micturition. An extreme hypospadial urethral duplication mimics a congenital urethroperineal fistula. In general, in a duplication the ventral urethra is the more functional channel, while with a fistula the dorsal channel is usually the primary one.

Cystography defines the underlying anatomy prior to surgical correction. For complete evaluation of a duplicated system, both a voiding cystourethrogram and a retrograde urethrogram are obtained.

Valves

Posterior valves

The most common cause of urethral obstruction in male infants is the presence of congenital posterior urethral valves. Obstruction varies in gradation. At times mild obstruction is not discovered until adolescence or even later. Some of these infants develop marked hydronephrosis and renal failure before the condition is detected. These valves are usually classified into three type:

Type I valves are folds extending from the verumontanum to the anterolateral urethral wall. These are most common.

Type II valves also arise from the verumontanum but extend proximally into the bladder neck. These valves tend to be of limited significance and probably are acquired rather than congenital in origin.

Type III valves consist of a prominent urogenital membrane with a central opening distal to the verumontanum. They are rare.

A syndrome exists of posterior urethral valves, persistent unilateral reflux, and unilateral renal dysplasia even to the point of nonfunction. A majority of infants and children with posterior urethral valves have vesicoureteral reflux, which often does not resolve after valve ablation. Long-term follow-up is thus required. Some of these infants also have impaired bladder compliance and detrusor instability, and these findings persist after valve